Heretofore, as an optical glass having high dispersibility, a phosphate optical glass containing P2O5 as the basic component has been known. However, the conventional phosphate optical glass has had a problem that it is poor in durability, particularly in chemical durability, and in a polishing step or washing step of the optical glass or during its use as a lens for a long period of time, weathering and erosion are likely to occur on its surface. Especially in the case of an optical glass to be subjected to precision press molding, the question of chemical durability becomes important, since it contains alkali metal/alkaline earth metal elements so that it can be easily molded.
As a proposal to improve the chemical durability of such phosphate optical glass, a composition containing a specific amount of Al2O3 has been proposed in Patent Document 1, but it is still not satisfactory from the point of chemical durability. Further, as a phosphate optical glass for precision press molding, a composition is disclosed also in e.g. Patent Document 2, but it is to cover a low dispersibility range of an Abbe number being from 23 to 42 and is not one to cover a high dispersibility range of an Abbe number being at least 65.
Patent Document 1: JP-A-57-118045 (detailed description of the invention)
Patent Document 2: JP-A-2003-238197 (detailed description of the invention)